Which email services support 2048 bit & 4096 bit RSA keys?

S/MIME and PGP keys generated by LuxSci use RSA. Customers can bring their own RSA keys or have us generate them for them. We use 2048 bit. There is really not much to be gained for going to 4096 bit. The curve of how easy it is to crack is not exponential in bit length but flattens off as the keys get bigger and bigger.